TOKYO (AP) — All Nippon Airways and Japan Airlines said they replaced lithium-ion batteries in their Boeing 787 Dreamliners on multiple occasions before a battery overheating incident led to the worldwide grounding of the jets.

ANA said Wednesday it replaced batteries on its 787 aircraft some 10 times because they failed to charge properly or showed other problems, and informed Boeing about the swaps. Japan Airlines said it had also replaced lithium-ion batteries on its 787 jets but couldn’t immediately give details.

All 50 of the Boeing 787s in use around the world were grounded after an ANA flight on Jan. 16 made an emergency landing in Japan when its main battery overheated. Earlier in January, a battery in a Japan Airlines 787 caught fire while parked at Boston’s Logan International Airport. Lithium-ion batteries are prone to overheating and require additional safeguards to prevent fires.

ANA spokeswoman Megumi Tezuka said the airline was not required to report the battery replacements to Japan’s Transport Ministry because they did not interfere with flights and did not raise safety concerns. She said that having to replace batteries on aircraft is not uncommon and that it was not considered out of the ordinary.

Laura Brown, a spokeswoman for the U.S. Federal Aviation Administration spokeswoman, said in Washington that the agency was checking whether the previous battery incidents had been reported by Boeing.

With 17 of the jets, ANA was Boeing’s launch customer for the technologically advanced airliner. The airline has had to cancel hundreds of flights, affecting tens of thousands of people, but has sought to minimize disruptions by switching to other aircraft as much as possible.

The battery problems experienced by ANA before the emergency landing were first reported by The New York Times.

Japanese and U.S. investigators looking into the Boeing 787’s battery problems shifted their attention this week from the battery-maker, GS Yuasa of Kyoto, Japan, to the manufacturer of a monitoring system. That company, Kanto Aircraft Instrument Co. makes a system that monitors voltage, charging and temperature of the lithium-ion batteries.

On Tuesday, the U.S. National Transportation Safety Board said it was conducting a chemical analysis of internal short circuiting and thermal damage of the battery that caught fire in Boston.

The probe is also analyzing data from flight data recorders on the aircraft, the NTSB said in a statement on its website.